A structure having a photoelectric conversion unit disposed outside a semiconductor substrate has been recently suggested as a technique for discontinuously changing the characteristics of an image sensor. For example, Patent Documents 1 through 3 each disclose a structure in which a photoelectric conversion unit is disposed in an upper portion of a semiconductor substrate, and photoelectrically converted signals are accumulated in the semiconductor substrate. In such a structure, photoelectric conversion characteristics that are conventionally determined by a semiconductor substrate material can be greatly changed. Such a structure might bring out a possibility that the sensor technology can be applied to the fields that are not easily realized with image sensors using conventional silicon (Si), such as use of far-infrared rays.
Also, in a pixel array in which red, blue, and green color filters that are widely used in today's image sensors are arranged in a two-dimensional manner, light of a certain wavelength is absorbed on a pixel-by-pixel basis, so that color separation is conducted. In a red pixel, for example, light of the wavelength of blue and green is absorbed by the color filter and is lost.
To counter this problem, Patent Document 1 suggests a stacked solid-state imaging device in which photoelectric conversion regions for photoelectrically converting red, blue, and green light are stacked in the same pixel space, for example. With this structure, decreases in sensitivity due to light absorption by color filters can be reduced. Furthermore, this structure does not need any interpolating, and therefore, an effect to avoid generation of false colors can be expected.
In a structure having a photoelectric conversion unit disposed outside a semiconductor substrate, a contact portion that electrically connects the photoelectric conversion unit and the semiconductor substrate is necessary. On the semiconductor substrate side, the contact portion is connected to an n-type diffusion layer surrounded by a p-type semiconductor, for example. This n-type diffusion layer functions as a charge retention unit that retains photoelectrically converted charge, but a buried PN junction cannot be formed due to the contact portion. As a result, leakage current is generated. For example, in a case where an n-type diffusion layer surrounded by a p-type semiconductor is used, a reverse bias leakage current of a PN junction is generated.